1. Field of the Invention
This invention relates generally to feedback control systems for controlling a free piston machine, and more particularly relates to apparatus and methods which are added to such control systems for the purpose of avoiding damaging collisions between the free piston and cylinder end structures, such as a cylinder head, against which the piston would, under some operating conditions, collide.
2. Description of the Related Art
Many types of machines utilize expansible chamber devices, such as a piston reciprocating in a cylinder, for operating upon gases. For example, they are used for pumping, compressing or displacing gases. Predominantly, pistons oscillate in linear reciprocation within a cylinder by means of a direct mechanical connection or linkage between a driver or a load and the piston. The linkage drives or is driven by the piston along a confined path of reciprocation and most commonly is a crankshaft and connecting rod system.
However, for some applications, such as high compression ratio compressors, free piston cryocoolers and free piston Stirling engines driving alternators, there are advantages to a free piston machine for such purposes. The piston is free because no mechanical linkage confines the piston to a fixed path of reciprocation. For example, a free piston may be driven by a linear, electric motor or a free piston Stirling engine. Typically, in order to maximize the efficient use of available drive power, free piston machines are driven at their frequency of mechanical resonance. Because the pistons are unconfined, the amplitude of reciprocation, may vary under the influence of changing operating conditions. Consequently, the piston, as well as any reciprocating structures attached to it, can collide at either end of the piston stroke with physical structures at the end of or beyond the cylinder.
In such freely reciprocating machines, the amplitude and frequency of reciprocation are a function of inertia, damping, and spring and driving forces. Therefore, these machines share the common feature that, when they are overdriven or underdamped, the reciprocating parts can acquire an amplitude of reciprocation that exceeds the internal geometrical limits of the space available for the motion of the reciprocating parts. If the amplitude of reciprocation is allowed to increase indefinently, the reciprocating parts will eventually collide repeatedly at the frequency of operation with stationary structures, or even with other reciprocating parts.
In free piston machines, it is always desirable to avoid collisions which have sufficient impact to damage the machine. However, under some operating conditions it is often desirable to maximize piston amplitude or stroke in order to maximize machine performance. Consequently, it is often desirable to operate the machine at the maximum amplitude which the machine can tolerate without damage to the machine. This requires accurate control of piston amplitude and rapid response to any changes in it.
A common practice in prior art free piston machines is to control one or more of the machine parameters, which affect or are affected by the forces applied by or to the piston, such as piston drive, by means of a conventional, closed-loop, negative feedback, control system. Such systems can be either analog or digitally controlled systems. Because piston amplitude is a function of piston forces, these systems exert some control over piston amplitude and assist in avoiding collision. However, because piston amplitude is a function of piston forces and some piston forces are a function of machine loading, and because machine loading can change, sometime rapidly, under varying operating conditions, such systems often do not permit operation at maximum amplitude of piston reciprocation because they cannot control piston amplitude with sufficient precision. Under large loading, the drive force can be very large, but the piston may not approach anywhere near a collision. However, reduction of the load can result in an increase in piston amplitude, permitting a collision to occur.
In a conventional feedback control system, including those applied to free piston machines, the control system compares a measured value with its desired value to produce an actuating error signal, which is acted upon to reduce the magnitude of the error. Referring to FIG. 1, a controlled system 10, such as the piston driver, for example a linear motor, is acted upon by a dynamic control element 12 having some preset forward transfer function. The control element 12 does the work of controlling the controlled system 10. For example, the control element 12 typically is a high gain amplifier. Historically the control element was an analog circuit, but its forward transfer control function can also be performed digitally by a microcontroller or discrete logic circuitry.
A feedback element 14, usually a sensor, applies a feedback signal to a summing junction 16 to provide the measured value. The feedback signal can be a function of piston amplitude, piston drive, piston displacement or other parameter which affects or is a function of piston amplitude.
One prior art system offering advantages in free piston machines is illustrated in U.S. Pat. No. 5,342,176, which is herein incorporated by reference. The command input 18, also referred to as the control input or reference input, provides the set point for the feedback control system and is summed with the feedback signal at the summing junction 16. This set point represents the desired value.
The command input may be a fixed quantity based upon the motor""s electromechanical transfer constant and a particular stroke. Alternatively, it can be a variable controlled independently or by an external, physical phenomenon, or by a control circuit that seeks to maintain a particular value of that external phenomenon. For example, the control input can be the output of a summing junction receiving inputs derived from a sensor measuring a temperature or pressure affected by the free piston machine, or from a temperature or pressure set point. Of course, the control input may, in its simplest form, be a manually input adjustment.
The output 20 of the summing junction 16 provides an error signal, which is applied to the dynamic control element 12. The error signal is the difference between the feedback signal and the reference input signal. Of course, the summing at the summing junction 16 may be performed in the more historical manner in an analog summing circuit, or alternatively it may also be performed digitally in a microcontroller or discrete logic circuitry. As known to those skilled in the art, not only may each of these elements of the control system and their signals be performed either in an analog or digital format, but hybrid systems, which include analog to digital converters and digital to analog converters, can also be constructed which utilize some of each mode. Consequently, the term xe2x80x9csumming junctionxe2x80x9d, as well as other control system terms, is not limited to analog circuits, but include digital implementations.
For purposes of describing the invention, the term xe2x80x9ccylinder end structurexe2x80x9d is used to refer to a physical body at either end of the linear path of piston reciprocation with which the piston, or structures linked to and oscillating with it, can collide if its amplitude of oscillation increases excessively. Most typically, this is a cylinder head in which valves are mounted. The term xe2x80x9cpiston drivexe2x80x9d or xe2x80x9cdrivexe2x80x9d is the driving force or power applied to the piston to force it in its reciprocating, linear oscillation. Since piston amplitude is an increasing function of piston drive, an increase or decrease of piston drive, respectively increases or decreases the amplitude of piston oscillation if other parameters remain constant or undergo only variations which do not completely negate the change in piston drive.
It is, therefore, an object and feature of the present invention to provide a control system for a free piston machine, which, under all operating conditions, avoids damaging or destructive collision of the piston, or component structures reciprocating with the piston, against cylinder end structures.
Another object of some embodiments of the invention is to provide a control system which not only avoids such collisions, but also reciprocates the piston near its maximum amplitude.
The invention recognizes that initial collisions of the piston or its associated reciprocating structures against cylinder end structures, are relatively gentle and can be tolerated because they are not damaging. However, they can be tolerated only if remedial measures are immediately taken to avoid further increase in piston amplitude and to stop such collisions. Otherwise the collisions will continue with progressively greater force eventually resulting in damage to one or more of the colliding parts. The rate of amplitude increase can be high enough within a few cycles of piston oscillation to become destructive and, therefore, remedial measures must be effective in less than a few cycles.
The invention detects an initial collision and any subsequent collisions and generates a change in a perturbation signal, which is also applied to the summing junction of the control system to reduce piston amplitude a sufficient amount to stop the collisions. Consequently, the invention is principally an addition to a feedback control system and can be implemented in several embodiments.
The invention has a collision detector for detecting a collision of the piston against a cylinder end structure and generating a signal at an output in response to a detected collision. The output of the collision detector is applied to a signal generator for generating a perturbation signal, with the output of the signal generator being connected to the summing junction.
In operating according to the method of the invention, the perturbation signal is summed with the feedback signal and the reference input signal of the control system. The magnitude of the perturbation signal is varied in a direction which reduces the piston amplitude in response to detection of such a collision, and this variation continues until no collisions are detected. Some embodiments of the invention utilize a hunting system in which, whenever no collisions are detected, the perturbation signal is varied in a direction to increase piston amplitude and whenever collisions are detected, the perturbation signal is varied in a direction to reduce piston amplitude. Consequently, in such an embodiment, the piston is always reciprocating near its maximum amplitude, yet destructive collisions are always avoided.
As with conventional control systems, the structures and methods of the present invention may be operated and performed in either an analog mode or a digital mode or hybrid combinations of the two. More specifically, the perturbation signal can be an analog signal, the magnitude of which is varied, or a digital signal for which the numerical value of the data represented by the digital signal is varied. The perturbation signal generator can be either analog or it can be digital in the form of a microcomputer or discrete logic circuits and analog/digital or digital/analog converters can be used in hybrid systems.
The term xe2x80x9cperturbation signalxe2x80x9d is used to refer to a digital or analog signal, which is algebraically summed with one or more other signals to cause changes in the other signal. In the present invention the perturbation signal is applied to the summing junction to change its output from what it would be in the absence of the perturbation signal. In the conventional feedback control system two additional signals are applied to the summing junction, so either could be thought of as being perturbed by the perturbation signal. However, it is most convenient to think of the perturbation signal as perturbing the reference input signal, that is perturbing the set point.